On the Internal Dynamics of Starless Cores: Stability of Starless Cores With Internal Motions and Collapse Dynamics

Abstract

In order to understand the collapse dynamics of observed low-mass starless cores, we revise the conventional stability condition of hydrostatic Bonnor-Ebert spheres to take internal motions into account. Because observed starless cores resemble Bonnor-Ebert density structures, the stability and dynamics of the starless cores are frequently analyzed by comparing to the conventional stability condition of a hydrostatic Bonnor-Ebert sphere. However, starless cores are not hydrostatic but have observed internal motions. In this study, we take gaseous spheres with a homologous internal velocity field and derive stability conditions of the spheres utilizing a virial analysis. We propose two limiting models of spontaneous gravitational collapse: the collapse of critical Bonnor-Ebert spheres and uniform density spheres. The collapse of these two limiting models are intended to provide the lower and the upper limits, respectively, of the infall speeds for a given density structure. The results of our study suggest that the stability condition sensitively depends on internal motions. A homologous inward motion with a transonic speed can reduce the critical size compared to the static Bonnor-Ebert sphere by more than a factor of two. As an application of the two limiting models of spontaneous gravitational collapse, we compare the density structures and infall speeds of the observed starless cores L63, L1544, L1689B, and L694-2 to the two limiting models. L1689B and L694-2 seem to have been perturbed to result in faster infall motions than for spontaneous gravitational collapse.

The effects of a compressive velocity pulse on a collapsing turbulent clump

“…It was mentioned in Section 1 that the infall velocities for the cores L1689B and L694-2 were observed to be faster than expected, see Lee et al (2007). A theoretical model proposed by Seo et al (2013), in which the collapse of the cores was modeled by means of a uniform density or as Bonner-Ebert spheres, demonstrated that these cores may have infall velocities up to -1.0 or -1.5 times the velocity normalized with the speed of sound, which indicates an enhanced collapse. Meanwhile, for the cores L1544 and L63, in which the collapse appeared to be normal, the magnitude of the infall velocity was around -0.5 times the speed of sound 4 .…”

“…The magnitudes of the infall velocities reported here have been taken from fig.6ofSeo et al (2013).Copyright line will be provided by the publisher…”

JCMT POL-2 and BISTRO Survey observations of magnetic fields in the L1689 molecular cloud

“…Redman et al 2002;Crapsi et al 2005;Bacmann et al 2016;Kim et al 2020), internal structure and dynamics (e.g. Lee et al 1999Lee et al , 2001Redman et al 2004;Seo et al 2013;Roy et al 2014) due to its relative isolation and simple morphology.…”

“…L1689B is a candidate gravitationally bound prestellar core. Its stability and age remain uncertain, with a number of studies identifying the core as contracting or showing signs of infall (Redman et al 2004;Sohn et al 2007;Lee & Myers 2011;Seo et al 2013), while Schnee et al (2013) found it to be static. Redman et al (2002) found L1689B to be relatively long-lived, with an age of at least one freefall time inferred from CO freezeout in the core's center; however, Lee et al (2003) argued that the core is chemically young, with a lack of freezeout of HCO+ and also a potential lack of CO freezeout.…”

“…This rotation axis is similar to the rotation axis in SMM-16, and the core is rotating in the same sense. Seo et al (2013) found infall velocities in L1689B greater than could be caused by gravitational collapse, and so inferred that core collapse has been instigated by some sort of exter-nal perturbation, suggesting that turbulence or a sudden increase in external pressure might be responsible.…”